One of the most dangerous maneuvers in driving in the United States is making a left turn. In the United States, traffic drives on the right-hand side of the road and on-coming traffic drives on the left-hand side of the road. To make a left turn, a vehicle must cross the on-coming traffic.
In other countries, such as the United Kingdom, traffic drives on the left-hand side of the road and on-coming traffic drives on the right-hand side of the road. In those countries, a right turn is dangerous because it requires crossing on-coming traffic. Throughout the rest of this disclosure we will discuss left turns but a person of ordinary skill would readily recognize that the same concepts and techniques apply to right turns in countries with traffic conventions such as those in the United Kingdom.
A traditional method for reducing the danger of left turns, illustrated in FIG. 1, uses lanes that are dedicated to left turns and traffic lights that include lights that regulate left turns. FIG. 1 shows an intersection between two four-lane roads. An intersection can be two roads crossing at substantially right angles, or it can be one road teeing into another, or it can be two or more roads coming together at an angle, such as in a Y intersection. A road can have any number of lanes and can have any surface.
A road may allow travel in two substantially opposite directions although a road can also be one-way. In FIG. 1, one road has two A lanes for travel in the A direction, i.e., from left to right on the page, and two lanes for travel in the A-prime direction, i.e., from right to left on the page. A second road that intersects with the first road has two B lanes for travel in the B direction, i.e., from top to bottom on the page, and two B-prime lanes for travel in the B-prime direction, i.e., from bottom to top on the page. For simplicity of discussion, it will be assumed for all of the drawings of intersections discussed herein that the top of the page is the north as indicated by the compass included on some or all of the Figure pages. Further, the “arm” of an intersection extending in a particular direction will be referred to in that manner (e.g., the “east arm” referring to the arm of the intersection extending to the right, or east, on the page). However, it will be understood that the orientation of the roads with respect to the compass is not limiting. For example, FIG. 1 may be oriented such that north on the compass is at the top of the drawing or it may be to the right of the drawing or any other orientation.
In the traditional approach illustrated in FIG. 1, each set of lanes is provided with a right-turn lane. For example, a vehicle would use right-turn lane 105 to turn from the A lanes to the B lanes. Similarly, each set of lanes is provided with a left-turn lane. For example, a vehicle would use left-turn lane 110 to turn from the A lanes to the B-prime lanes.
In addition, in the traditional approach illustrated in FIG. 1 the intersection may be controlled by traffic indicators, such as traffic lights. A different set of traffic lights may be devoted to some or all of the directions that traffic can travel through the intersection. Each direction may have the traditional red (for stop), yellow (for caution), and green (for proceed) lights, each color representing a state of the traffic flow. A turn may have a green arrow, indicating that the turn can be made, instead of the traditional green light. Similarly, the yellow and red lights for a turn can be replaced with yellow and red arrows, respectively. As an alternative to lights, the states may be represented by mechanical indicators, such as wooden, metal, or cloth flags. The traffic indicators may be attached to a pole adjacent the road or they may be suspended from a cable strung across the intersection.
The traffic indicators may have phases. A traffic indicator “phase” defines the state of the traffic indicators at an intersection. For example, one phase may be defined by the left turn indicator for the turn from the A lanes to the B-prime lanes being green and all other lights being red. Another phase may be for the lights over the A lanes and the A-prime lanes to be green and all other lights to be red. Each phase may define a set of one or more protected paths through an intersection. A “protected path” is one in which traffic has right of way superior to that of traffic following intersecting paths. For example, an intersection between two bi-directional roads may have four phases, such as those illustrated in FIGS. 1A-D. In the first and third phases (FIGS. 1A and 1C), left turns are protected paths and through traffic is not allowed. In the second and fourth phases (FIGS. 1B and 1D), through traffic lanes are protected paths and left turns are not protected paths (and may not be allowed). A “cycle” is a progression through the phases of the one or more traffic indicators. In some cases of “running through a cycle,” a phase may be skipped. For example, in the intersection illustrated in FIG. 1, a detector may be situated to detect whether traffic is waiting to turn left. If there is no such traffic one or more of the left-turn phases may be skipped.